Multi-engine coordination during gas turbine engine motoring

ABSTRACT

A system is provided for multi-engine coordination of gas turbine engine motoring in an aircraft. The system includes a controller operable to determine a motoring mode as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on at least one temperature of a plurality of gas turbine engines and initiate dry motoring based on the motoring mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional Application No. 62/365,093, filed Jul. 21, 2016, which is incorporated herein by reference in its entirety.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly to systems and methods for multi-engine coordination during gas turbine engine motoring.

Gas turbine engines are used in numerous applications, one of which is for providing thrust to an airplane. When the gas turbine engine of an airplane has been shut off for example, after an airplane has landed at an airport, the engine is hot and due to heat rise, the upper portions of the engine will be hotter than lower portions of the engine. When this occurs thermal expansion may cause deflection of components of the engine which may result in a “bowed rotor” condition. If a gas turbine engine is in such a bowed rotor condition it is undesirable to restart or start the engine.

One approach to mitigating a bowed rotor condition is to use a starter system to drive rotation of a spool within the engine for an extended period of time at a speed below which a resonance occurs (i.e., a critical speed or frequency) that may lead to damage when a sufficiently large bowed rotor condition is present. Motoring is typically performed separately for each engine at different times, which extends the total start time for a multi-engine aircraft.

BRIEF DESCRIPTION

In an embodiment, a system is provided for multi-engine coordination of gas turbine engine motoring in an aircraft. The system includes a controller operable to determine a motoring mode as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on at least one temperature of a plurality of gas turbine engines and initiate dry motoring based on the motoring mode.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the at least one temperature is a measured core engine temperature or an oil temperature.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where dry motoring is inhibited when the aircraft is not on the ground.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the motoring mode is further determined based on a plurality of performance parameters that are based on one or more of: an ambient condition, performance limitations of a compressed air source and each air turbine starter driven by the compressed air source, engine drag, and parasitic factors.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the performance parameters are determined based on one or more of: an ambient air temperature, an ambient pressure, and an oil temperature.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the compressed air source is an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller is further operable to monitor a speed of each of the gas turbine engines when dry motoring is active and switch from the multi-engine dry motoring mode to the single engine dry motoring mode based on one or more of the gas turbine engines failing to reach or maintain a dry motoring threshold speed for a predetermined time limit.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the dry motoring threshold speed is dynamically adjusted to match a dry motoring profile.

According to an embodiment, a system of an aircraft includes a compressed air source operable to supply compressed air, a first engine system, a second engine system, and at least one engine control interface. The first engine system includes a first gas turbine engine, a first air turbine starter, a first starter air valve, and a first controller operable to command the first starter air valve to control delivery of the compressed air to the first air turbine starter during motoring of the first gas turbine engine. The second engine system includes a second gas turbine engine, a second air turbine starter, a second starter air valve, and a second controller operable to command the second starter air valve to control delivery of the compressed air to the second air turbine starter during motoring of the second gas turbine engine. The at least one engine control interface is operable to determine a motoring mode as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on at least one temperature of the first and second gas turbine engines and initiate dry motoring based on the motoring mode.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the at least one engine control interface is further operable to monitor a speed of the first and second gas turbine engines when dry motoring is active and switch from the multi-engine dry motoring mode to the single engine dry motoring mode based the first or second gas turbine engine failing to reach or maintain a dry motoring threshold speed for a predetermined time limit.

Another embodiment includes a method for multi-engine coordination during gas turbine engine motoring. The method includes receiving, by a controller, at least one temperature of a plurality of gas turbine engines. The controller determines a motoring mode as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on the at least one temperature of the plurality of gas turbine engines. Dry motoring is initiated based on the motoring mode.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include monitoring a speed of each of the gas turbine engines when dry motoring is active, and switching from the multi-engine dry motoring mode to the single engine dry motoring mode based on one or more of the gas turbine engines failing to reach or maintain a dry motoring threshold speed for a predetermined time limit.

A technical effect of the apparatus, systems and methods is achieved by using coordinated multi-engine control for bowed rotor mitigation of gas turbine engines as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of an aircraft engine starting system in accordance with an embodiment of the disclosure;

FIG. 2 is another schematic illustration of an aircraft engine starting system in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a high spool gas path with a straddle-mounted spool in accordance with an embodiment of the disclosure;

FIG. 4 is a schematic illustration of a high spool gas path with an overhung spool in accordance with an embodiment of the disclosure;

FIG. 5 is a block diagram of a system for bowed rotor start mitigation in accordance with an embodiment of the disclosure;

FIG. 6 is a block diagram of mode selection for single or multi-engine dry motoring control in accordance with an embodiment of the disclosure; and

FIG. 7 is a flow chart illustrating a method in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowed rotor start mitigation system in a gas turbine engine. Embodiments can include using a starter air valve to control a rotor speed of a starting spool of a gas turbine engine to mitigate a bowed rotor condition using a dry motoring process. During dry motoring, the starter air valve can be actively adjusted to deliver air pressure (i.e., compressed air) from an air supply to an air turbine starter of an engine starting system that controls starting spool rotor speed. Dry motoring may be performed by running an engine starting system at a lower speed with a longer duration than typically used for engine starting while dynamically adjusting the starter air valve to maintain the rotor speed and/or follow a dry motoring profile. Some embodiments increase the rotor speed of the starting spool to approach a critical rotor speed gradually and as thermal distortion is decreased they then accelerate beyond the critical rotor speed to complete the engine starting process. The critical rotor speed refers to a major resonance speed where, if the temperatures are unhomogenized, the combination of a bowed rotor and similarly bowed casing and the resonance would lead to high amplitude oscillation in the rotor and high rubbing of blade tips on one side of the rotor, especially in the high pressure compressor if the rotor is straddle-mounted.

A dry motoring profile for dry motoring can be selected based on various parameters, such as a modeled temperature value of the gas turbine engine used to estimate heat stored in the engine core when a start sequence is initiated and identify a risk of a bowed rotor. The modeled temperature value alone or in combination with other values (e.g., measured temperatures) can be used to calculate a bowed rotor risk parameter. For example, the modeled temperature can be adjusted relative to an ambient temperature when calculating the bowed rotor risk parameter. The bowed rotor risk parameter may be used to take a control action to mitigate the risk of starting the gas turbine engine with a bowed rotor. The control action can include dry motoring consistent with the dry motoring profile. In some embodiments, a targeted rotor speed profile of the dry motoring profile can be adjusted as dry motoring is performed.

A full authority digital engine control (FADEC) system or other system may send a message to the cockpit to inform the crew of an extended time start time due to bowed rotor mitigation actions prior to completing an engine start sequence. If the engine is in a ground test or in a test stand, a message can be sent to the test stand or cockpit based on the control-calculated risk of a bowed rotor. A test stand crew can be alerted regarding a requirement to keep the starting spool of the engine to a speed below the known resonance speed of the rotor in order to homogenize the temperature of the rotor and the casings about the rotor which also are distorted by temperature non-uniformity.

In order to further reduce total start time, embodiments enable dry motoring of multiple engines at the same time. Respective FADECs for each engine can provide parameters to one or more engine control interfaces including sensed temperatures and other values that may impact mode selection decisions for enabling single engine or multi-engine cranking for dry motoring. An engine control interface that receives the parameters can determine present conditions with respect to an operating envelope of a compressed air source and the starting system of each engine based on predetermined performance constraints, engine drag, and/or parasitic factors. Multi-engine cranking (e.g., dual engine cranking) can be commanded to perform dry motoring in multiple engines at the same time based on determining that each of the starting systems will receive a sufficient supply of compressed air for the engines to reach a targeted speed based on the present conditions. The engine control interface can continue monitoring performance of the engines during dry motoring and switch to single engine cranking upon determining that the engines are not reaching the targeted speed for a predetermined period of time.

Referring now to FIG. 1, a schematic illustration of an aircraft 5 is depicted with a pair of engine systems 100A, 100B. Engine systems 100A, 100B include gas turbine engines 10A, 10B and engine starting systems 101A, 101B respectively. Engine systems 100A, 100B also include FADECs 102A, 102B to control gas turbine engines 10A, 10B and starting systems 101A, 101B. FADECs 102A, 102B may generally be referred to as controllers. FADECs 102A, 102B can communicate with respective engine control interfaces 105A, 105B using a digital communication bus 106. The engine control interfaces 105A, 105B can buffer engine system communication from aircraft level communication. Although depicted separately in FIG. 1, in some embodiments the engine control interfaces 105A, 105B are integrated with the FADECs 102A, 102B. The engine control interfaces 105A, 105B may also be referred to as controllers when configured to make mode selection determinations to perform single engine or multi-engine dry motoring for the aircraft 5.

In an embodiment, the FADECs 102A, 102B and engine control interfaces 105A, 105B may each include memory to store instructions that are executed by one or more processors on one or more channels. The executable instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with a controlling and/or monitoring operation of the gas turbine engines 10A, 10B of FIG. 1. The one or more processors can be any type of central processing unit (CPU), including a general purpose processor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Also, in embodiments, the memory may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and control algorithms in a non-transitory form.

In the example of FIG. 1, an auxiliary power unit (APU) 113 and compressor 115 provide a compressed air source 114 to drive air turbine starters 120A, 120B of engine starting systems 101A, 101B. Compressed air from the compressed air source 114 is routed through ducts 117 and air starter valves 116A, 116B to the air turbine starters 120A, 120B. Various shutoff valves can also be included in ducts 117, such as a main shutoff valve 119 and engine shutoff valves 121A, 121B. One or more bleed valves 123 can be used to release compressed air from the ducts 117.

In some cases, dry motoring can be performed simultaneously for engine systems 100A, 100B, where compressed air from the compressed air source 114 is provided to both air turbine starters 120A, 120B at the same time. When one of the engine systems 100A, 100B completes dry motoring before the other, a disturbance or pressure surge of compressed air may be experienced at the starter air valve 116A, 116B and air turbine starter 120A, 120B of the engine system 100A, 100B still performing dry motoring. The FADECs 102A, 102B can be configured with control laws to maintain a motoring speed below a threshold level (i.e., the critical rotor speed) for the engine system 100A, 100B while performing dry motoring based on compressed air source 114. In embodiments, FADECs 102A, 102B can observe various engine parameters and starting system parameters to actively control dry motoring and prevent fault conditions from damaging the gas turbine engines 10A, 10B. For example, FADECs 102A, 102B can observe engine speeds (N2) of gas turbine engines 10A, 10B and may receive starter system parameters such as starter speeds (NS) and/or starter air pressures (SAP). In embodiments, FADECs 102A, 102B can adjust starter air valves 116A, 116B based on measured feedback to reject disturbances attributable to dual to single or single to dual engine starting transitions, such as switching from dry motoring both gas turbine engines 10A, 10B to dry motoring either gas turbine engine 10A or 10B.

To further enhance control aspects, the FADECs 102A, 102B can provide either or both of the engine control interfaces 105A, 105B with engine data including parameters that directly or indirectly modify an aspect of the compressed air received at the starter air valves 116A, 116B. Engine data can be sent on the digital communication bus 106 to either or both of the engine control interfaces 105A, 105B to make single engine/multi-engine cranking determinations. Engine data may include fault information, such as a detected failure of the starter air valves 116A, 116B and/or the air turbine starters 120A, 120B. Present condition information and/or commands included in the engine data can allow the engine control interfaces 105A, 105B to track and/or predict events that will impact available compressed air for dry motoring at each of the engine starting systems 101A, 101B. For example, at least one temperature of gas turbine engines 10A, 10B, such as a measured core engine temperature or an oil temperature, can be used to determine current conditions and select a multi-engine dry motoring envelope which can be used to identify present multi-engine dry motoring capability (e.g., whether a dry motoring threshold speed can be reached by two or more engines 10A, 10B at the same time). Additional performance parameters, such as an ambient air temperature and/or an ambient pressure, can also be used to determine whether a dry motoring threshold speed can be reached. For instance, ambient temperature can be used for temperature comparison/normalization and ambient pressure can be used to adjust for altitude effects.

Although FIG. 1 depicts one example configuration, it will be understood that embodiments as described herein can cover a wide range of configurations, such as a four engine system. Further, the compressed air source 114 can include multiple sources other than APU 113 and compressor 115, such as a ground cart or cross engine bleed air.

Turning now to FIG. 2, a schematic of engine systems 100A, 100B and engine starting systems 101A, 101B for the gas turbine engines 10A, 10B of FIG. 1 are depicted according to an embodiment. In the example of FIG. 2, the digital communication bus 106 can include an aircraft, engine, and/or test stand communication bus to interface with FADECs 102A, 102B, engine control interfaces 105A, 105B, aircraft controls, e.g., a cockpit, various onboard computer systems, and/or a test stand (not depicted). Either or both channels of FADECs 102A, 102B can alternate on and off commands to respective electromechanical devices 110A, 110B coupled to starter air valves 116A, 116B to achieve a partially open position of the starter air valves 116A, 116B to control a flow of compressed air from compressed air source 114 (e.g., APU 113 and compressor 115 of FIG. 1) as a starter air flow to air turbine starters 120A, 120B during dry motoring. The air turbine starters 120A, 120B output torque to drive rotation of respective gas turbine engine shafts 50A, 50B of starting spools of the gas turbine engines 10A, 10B.

The FADECs 102A, 102B can monitor engine speed (N2), starter speed (NS), starter air pressure (SAP), and/or other engine parameters to determine an engine operating state and control the starter air valves 116A, 116B. Thus, the FADECs 102A, 102B can each establish a control loop with respect to a motoring speed (N2 and/or NS) and/or starter air pressure to adjust positioning of the starter air valves 116A, 116B. The FADECs 102A, 102B can also transmit engine data on digital communication bus 106 to engine control interfaces 105A, 105B, including present conditions and commands of each engine system 100A, 100B that may impact characteristics of the compressed air available at the starter air valves 116A, 116B.

In some embodiments, the starter air valves 116A, 116B are discrete valves designed as on/off valves that are typically commanded to either fully opened or fully closed. However, there is a time lag to achieve the fully open position and the fully closed position. By selectively alternating an on-command time with an off-command time through the electromechanical devices 110A, 110B, intermediate positioning states (i.e., partially opened/closed) can be achieved. The FADECs 102A, 120B can modulate the on and off commands (e.g., as a duty cycle using pulse width modulation) to the electromechanical devices 110A, 110B to further open the starter air valves 116A, 116B and increase a rotational speed of the gas turbine engine shafts 50A, 50B. In an embodiment, the electromechanical devices 110A, 110B have a cycle time defined between an off-command to an on-command to the off-command that is at most half of a movement time for the starter air valves 116A, 116B to transition from fully closed to fully open. Pneumatic lines or mechanical linkage (not depicted) can be used to drive the starter air valves 116A, 116B between the open position and the closed position. The electromechanical devices 110A, 110B can each be a solenoid that positions the starter air valves 116A, 116B based on intermittently supplied electric power as commanded by the FADECs 102A, 102B. In an alternate embodiment, the electromechanical devices 110A, 110B are electric valves controlling muscle air to adjust the position of the starter air valves 116A, 116B as commanded by the FADECs 102A, 102B.

In an alternate embodiment, rather than using electromechanical devices 110A, 110B to achieve a partially open position of the starter air valves 116A, 116B, the starter air valves 116A, 116B can be variable position valves that are dynamically adjustable to selected valve angles by the FADECs 102A, 102B. When implemented as variable position valves, the starter air valves 116A, 116B can be continuous/infinitely adjustable and hold a commanded valve angle, which may be expressed in terms of a percentage open/closed and/or an angular value (e.g., degrees or radians). Performance parameters of the starter air valves 116A, 116B can be selected to meet dynamic response requirements. For example, in some embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 40 seconds. In other embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 30 seconds. In further embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 20 seconds.

In some embodiments, the FADECs 102A, 102B can each monitor a valve angle of the starter air valves 116A, 116B when valve angle feedback is available. The FADECs 102A, 102B can establish an outer control loop with respect to motoring speed and an inner control loop with respect to the valve angle of the starter air valves 116A, 116B. Valve angle feedback and/or valve commands can be included in the cross engine data exchanged between the FADECs 102A, 102B.

FIGS. 3 and 4 depict two example engine configurations of the gas turbine engines 10A, 10B of FIG. 1. FIG. 3 is an example of a straddle-mounted spool 32A as a starting spool configuration. This configuration places two bearing compartments 37A and 39A (which may include a ball bearing and a roller bearing respectively), outside of the plane of most of the compressor disks of high pressure compressor 52A and at outside at least one of the turbine disks of high pressure turbine 54A. In contrast with a straddle-mounted spool arrangement, other embodiments may be implemented using an over-hung mounted spool 32B as depicted in FIG. 4 as a starting spool configuration. In over-hung mounted spool 32B, a bearing compartment 37B is located forward of the first turbine disk of high pressure turbine 54B such that the high pressure turbine 54B is overhung, and it is physically located aft of its main supporting structure. The use of straddle-mounted spools has advantages and disadvantages in the design of a gas turbine, but one characteristic of the straddle-mounted design is that the span between the bearing compartments 37A and 39A is long, making the amplitude of the high spot of a bowed rotor greater and the resonance speed that cannot be transited prior to temperature homogenization is lower. For any thrust rating, the straddle mounted arrangement, such as straddle-mounted spool 32A, gives Lsupport/Dhpt values that are higher, and the overhung mounted arrangement, such as overhung spool 32B, can be as much as 60% of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance between bearings (e.g., between bearing compartments 37A and 39A or between bearing compartments 37B and 39B), and Dhpt is the diameter of the last blade of the high pressure turbine (e.g., high pressure turbine 54A or high pressure turbine 54B). As one example, a straddle-mounted engine starting spool, such as straddle-mounted spool 32A, with a roller bearing at bearing compartment 39A located aft of the high pressure turbine 54A may be more vulnerable to bowed rotor problems since the Lsupport/Dhpt ranges from 1.9 to 5.6.

FIGS. 3 and 4 also illustrate an air turbine starter 120 (e.g., air turbine starter 120A or 120B of FIGS. 1 and 2) interfacing through gearbox 124 via a tower shaft 55 with the straddle-mounted spool 32A proximate high compressor 52A and interfacing via tower shaft 55 with the overhung mounted spool 32B proximate high compressor 52B as part of a starting system. The straddle-mounted spool 32A and the over-hung mounted spool 32B are both examples of a starter spool having a gas turbine engine shaft 50 driven by the air turbine starter 120, such as gas turbine engine shafts 50A, 50B driven by air turbine starters 120A, 120B of FIG. 2.

FIG. 5 is a block diagram of a system 200 for bowed rotor start mitigation that may control either of the starter air valves 116A, 116B of FIGS. 1 and 2 via control signals 210 in accordance with an embodiment. The system 200 may also be referred to as a bowed rotor start mitigation system. In the example of FIG. 5, the system 200 includes an onboard model 202 operable to produce a compressor exit temperature T₃ and a compressor inlet flow W₂₅ of one of the gas turbine engines 10A, 10B of FIG. 1 for use by a core temperature model 204. The onboard model 202 is configured to synthesize or predict major temperatures and pressures throughout one of the gas turbine engines 10A, 10B of FIG. 1 beyond those sensed by sensors positioned about the gas turbine engines 10A, 10B. The onboard model 202 and core temperature model 204 are examples of a first thermal model and a second thermal model that may be separately implemented or combined as part of a controller 102 (e.g., FADECs 102A, 102B of FIG. 1).

Engine parameter synthesis is performed by the onboard model 202, and the engine parameter synthesis may be performed using the technologies described in U.S. Patent Publication No. 2011/0077783, the entire contents of which are incorporated herein by reference thereto. Of the many parameters synthesized by onboard model 202 at least two are outputted to the core temperature model 204, T₃, which is the compressor exit gas temperature of each gas turbine engine 10A, 10B and W₂₅, which is the air flow through the compressor. Each of these values are synthesized by onboard model 202 and inputted into the core temperature model 204 that synthesizes or provides a heat state (T_(core)) of each gas turbine engine 10A, 10B. T_(core) can be determined by a first order lag or function of T₃ and a numerical value X (e.g., f(T₃, X)), wherein X is a value determined from a lookup table stored in memory of controller 102. Accordingly, X is dependent upon the synthesized value of W₂₅. In other words, W₂₅ when compared to a lookup table of the core temperature model 204 will determine a value X to be used in determining the heat state or T_(core) of each gas turbine engine 10A, 10B. In one embodiment, the higher the value of W₂₅ or the higher the flow rate through the compressor the lower the value of X.

The heat state of each engine 10A, 10B during use or T_(core) is determined or synthesized by the core temperature model 204 as each engine 10A, 10B is being run. In addition, T₃ and W₂₅ are determined (e.g., measured) or synthesized by the onboard model 202 and/or the controller 102 as each engine 10A, 10B is being operated.

At engine shutdown, the current or most recently determined heat state of the engine or T_(core shutdown) of an engine 10A, 10B is recorded into data storage unit (DSU) 104, and the time of the engine shutdown t_(shutdown) is recorded into the DSU 104. The DSU 104 retains data between shutdowns using non-volatile memory. Each engine 10A, 10B may have a separate DSU 104. Time values and other parameters may be received on digital communication bus 106. As long as electrical power is present for the controller 102 and DSU 104, additional values of temperature data may be monitored for comparison with modeled temperature data to validate one or more temperature models (e.g., onboard model 202 and/or core temperature model 204) of each gas turbine engine 10A, 10B.

During an engine start sequence or restart sequence, a bowed rotor start risk model 206 (also referred to as risk model 206) of the controller 102 is provided with the data stored in the DSU 104, namely T_(core shutdown) and the time of the engine shutdown t_(shutdown). In addition, the bowed rotor start risk model 206 is also provided with the time of engine start t_(start) and the ambient temperature of the air provided to the inlet of each engine 10A, 10B T_(inlet) or T₂. T₂ is a sensed value as opposed to the synthesized value of T₃ in some embodiments. In some embodiments, an oil temperature (T_(oil)) is a sensed value that can be used to determine a current temperature in combination with T_(core) and/or T₂. For instance, once oil stops circulating at shutdown, T_(oil) can provide a localized temperature reading indicative of a bearing compartment temperature from which temperatures at various engine locations can be derived.

The bowed rotor start risk model 206 maps core temperature model data with time data and ambient temperature data to establish a motoring time t_(motoring) as an estimated period of motoring to mitigate a bowed rotor of each gas turbine engine 10A, 10B. The motoring time t_(motoring) is indicative of a bowed rotor risk parameter computed by the bowed rotor start risk model 206. For example, a higher risk of a bowed rotor may result in a longer duration of dry motoring to reduce a temperature gradient prior to starting each gas turbine engine 10A, 10B of FIG. 1. In one embodiment, an engine start sequence may automatically include a modified start sequence; however, the duration of the modified start sequence prior to a normal start sequence will vary based upon the time period t_(motoring) that is calculated by the bowed rotor start risk model 206. The motoring time t_(motoring) for predetermined target speed N_(target) of each engine 10A, 10B is calculated as a function of T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil), (e.g., f(T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil)), while a target speed N_(target) is a predetermined speed that can be fixed or vary within a predetermined speed range of N_(targetMin) to N_(targetMax). In other words, the target speed N_(target) may be the same regardless of the calculated time period t_(motoring) or may vary within the predetermined speed range of N_(targetMin) to N_(targetMax). The target speed N_(target) may also be referred to as a dry motoring mode speed.

Based upon these values (T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil)) the motoring time t_(motoring) at a predetermined target speed N_(target) for the modified start sequence of each engine 10A, 10B is determined by the bowed rotor start risk model 206. Based upon the calculated time period t_(motoring) which is calculated as a time to run each engine 10A, 10B at a predetermined target speed N_(target) in order to clear a “bowed condition”. In accordance with an embodiment of the disclosure, the controller 102 can run through a modified start sequence upon a start command given to each engine 10A, 10B by an operator of the engines 10A, 10B, such as a pilot of an airplane the engines 10A, 10B are used with. It is understood that the motoring time t_(motoring) of the modified start sequence may be in a range of 0 seconds to minutes, which depends on the values of T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil).

In an alternate embodiment, the modified start sequence may only be run when the bowed rotor start risk model 206 has determined that the motoring time t_(motoring) is greater than zero seconds upon receipt of a start command given to each engine 10A, 10B. In this embodiment and if the bowed rotor start risk model 206 has determined that t_(motoring) is not greater than zero seconds, a normal start sequence will be initiated upon receipt of a start command to each engine 10A, 10B.

Accordingly and during an engine command start, the bowed rotor start risk model 206 of the system 200 may be referenced wherein the bowed rotor start risk model 206 correlates the elapsed time since the last engine shutdown time and the shutdown heat state of each engine 10A, 10B as well as the current start time t_(start) and the inlet air temperature T₂ in order to determine the duration of the modified start sequence wherein motoring of each engine 10A, 10B at a reduced speed N_(target) without fuel and ignition is required. As used herein, motoring of each engine 10A, 10B in a modified start sequence refers to the turning of a starting spool by air turbine starter 120A, 120B at a reduced speed N_(target) without introduction of fuel and an ignition source in order to cool the engine 10A, 10B to a point wherein a normal start sequence can be implemented without starting the engine 10A, 10B in a bowed rotor state. In other words, cool or ambient air is drawn into the engine 10A, 10B while motoring the engine 10A, 10B at a reduced speed in order to clear the “bowed rotor” condition, which is referred to as a dry motoring mode.

The bowed rotor start risk model 206 can output the motoring time t_(motoring) to a motoring controller 208. The motoring controller 208 uses a dynamic control calculation in order to determine a required valve position of the starter air valve 116A, 116B used to supply an air supply or compressed air source 114 to the engine 10A, 10B in order to limit the motoring speed of the engine 10A, 10B to the target speed N_(target) due to the position of the starter air valve 116A, 116B. The required valve position of the starter air valve 116A, 116B can be determined based upon an air supply pressure as well as other factors including but not limited to ambient air temperature, parasitic drag on the engine 10A, 10B from a variety of engine driven components such as electric generators and hydraulic pumps, and other variables such that the motoring controller 208 closes the loop for an engine motoring speed target N_(target) for the required amount of time based on the output of the bowed rotor start risk model 206. Engine data 209 can also be transmitted on digital communication bus 106 to the engine control interfaces 105A, 105B including present conditions, commands, and/or scheduled adjustments of the starting system 101A, 101B to assist the engine control interfaces 105A, 105B in making cranking mode determinations as a single/multi engine dry motoring mode 205 (also referred to as motoring mode 205). For instance, engine data 209 can include a measured core engine temperature, an oil temperature, an ambient air temperature, an ambient pressure, a starting spool speed, and the like. Engine data 209 can also include a number of control parameters such as a dry motoring profile and/or adjustments to a dry motoring profile. The single/multi engine dry motoring mode 205 can be used to pause/delay dry motoring and/or change motoring target speed. Changes may include extending the motoring time requirements predicted by the bowed rotor start risk model 206 and/or modifying a selection or applying a rescaling to a dry motoring profile determined by the bowed rotor start risk model 206.

In one embodiment, the dynamic control of the valve position (e.g., open state of the valve (e.g., fully open, ½ open, ¼ open, etc.) in order to limit the motoring speed of the engine 10A, 10B) is controlled via duty cycle control (on/off timing using pulse width modulation) of electromechanical device 110A, 110B for starter air valves 116A, 116B. When variable position starter air valves are used as the starter air valves 116A, 116B, a valve angle 207 can be provided to motoring control 208 based on valve angle feedback. A rotor speed N2 can be provided to the motoring controller 208 and a mitigation monitor 214, where motoring controller 208 and a mitigation monitor 214 may be part of controller 102.

The risk model 206 can determine a bowed rotor risk parameter that is based on the heat stored (T_(core)) using a mapping function or lookup table. When not implemented as a fixed rotor speed, the bowed rotor risk parameter can have an associated dry motoring profile defining a target rotor speed profile over an anticipated amount of time for the motoring controller 208 to send control signals 210, such as valve control signals for controlling starter air valves 116A, 116B of FIG. 3.

The bowed rotor risk parameter may be quantified according to a profile curve selected from a family of curves that align with observed aircraft/engine conditions that impact turbine bore temperature and the resulting bowed rotor risk. In some embodiments, an anticipated amount of dry motoring time can be used to determine a target rotor speed profile in a dry motoring profile for the currently observed conditions. As one example, one or more baseline characteristic curves for the target rotor speed profile can be defined in tables or according to functions that may be rescaled to align with the observed conditions.

In summary with reference to FIG. 5, as one example of an aircraft 5 that includes systems as described herein, onboard model 202 and core temperature model 204 may run on controller 102 of the aircraft 5 to track heat stored (T_(core)) in the turbine at the time of engine shutdown. Modeling of potential heat stored in the system may be performed as a turbine disk metal temperature model in the core temperature model 204. When the aircraft lands, engines typically operate at idle for a cool down period of time, e.g., while taxiing to a final destination. When an engine shutdown is detected, model state data can be logged by the DSU 104 prior to depowering. When the controller 102 powers on at a later time and model state data can be retrieved from the DSU 104, and the bowed rotor start risk model 206 can be updated to account for the elapsed time. When an engine start is requested, a bowed rotor risk can be assessed with respect to the bowed rotor start risk model 206. Extended dry motoring can be performed during an engine start process until the bow risk has sufficiently diminished. The state of or changes to the single/multi engine dry motoring mode 205 can start/stop dry motoring and/or result in adjustments to motoring time and/or a dry motoring profile used to drive control signals 210.

In reference to FIG. 5, the mitigation monitor 214 can operate in response to receiving a complete indicator 212 to run a verification of the bowed rotor mitigation. The mitigation monitor 214 can provide mitigation results 216 to the motoring controller 208 and may provide result metrics 218 to other systems, such a maintenance request or indicator. The mitigation monitor 214 may also run while dry motoring is active to determine whether adjustments to the dry motoring profile are needed. If the mitigation monitor 214 determines that a bowed rotor condition still exists, the motoring controller 208 may restart dry motoring, or a maintenance request or indicator can be triggered along with providing result metrics 218 for further analysis. Metrics of attempted bowed rotor mitigation can be recorded in the DSU 104 based on determining that the attempted bowed rotor mitigation was unsuccessful or incomplete. Mitigation results 216 and/or result metrics 218 may also be included in the engine data 209 sent to engine control interfaces 105A, 105B of FIG. 1.

FIG. 6 is a block diagram of mode selection logic 250 for single or multi-engine dry motoring control in accordance with an embodiment. With respect to the description of FIG. 6, engine system 100A is referred to as a first engine system 100A, while engine system 100B is referred to as a second engine system 100B; however, it will be understood that the first/second designations can be reversed. Although inputs from only two engines are depicted in the example of FIG. 6, it will be understood that the design can scale to any number of engines (e.g., a four engine aircraft). The mode selection logic 250 may be part of either or both of the engine control interfaces 105A, 105B of FIG. 1.

In the example of FIG. 6, the mode selection logic 250 includes a mode controller 252 and a speed monitor 254. The mode controller 252 can receive engine data 209 from FADECs 102A, 102B including a first engine temperature value 256A and a second engine temperature value 256B. Various parameters can be received or otherwise determined by the engine control interfaces 105A, 105B, such as air turbine starter parameters 258, compressed air source parameters 260, engine drag 262, parasitic factors 264, and flight regime 266. The air turbine starter parameters 258 can include performance limitations of the air turbine starters 120A, 120B. The compressed air source parameters 260 can include performance limitations of the compressed air source 114, such as output limits as a function of pressure and/or temperature. The engine drag 262 may be determined as a function of oil temperature and/or ambient conditions for each of the engines 10A, 10B. The parasitic factors 264 can be defined as correlations with respect to one or more of an ambient temperature, engine core temperature, and/or oil temperature. The flight regime 266 can be used to determine when dry motoring should be enabled or inhibited. For instance, dry motoring can be inhibited when the aircraft 5 of FIG. 1 is not on the ground. The various parameters available to the mode controller 252 enable selection of an envelope map that corresponds with the current conditions to determine when the system is collectively capable of performing multi-engine dry motoring such that multiple gas turbine engines 10A, 10B can reach a targeted motoring speed or track a desired dry motoring profile at the same time. Results of the determination of the mode controller 252 are output as the single/multi engine dry motoring mode 205, for instance, to FADECs 102A, 102B.

The speed monitor 254 can also receive engine data 209, which may include a first engine speed (e.g., N2 of engine 10A) 270A, a first engine dry motoring enabled status 272A, a second engine speed (e.g., N2 of engine 10B) 270B, and a second engine dry motoring enabled status 272B. When dry motoring is active in engines 10A, 10B (e.g., as determined based on the first and second engine dry motoring enabled status 272A, 272B), the speed monitor 254 can monitor the speed (e.g., first and second engine speed 270A, 270B) of each of the engines 10A, 10B. The speed monitor 254 can send a switch to single engine mode command 274 to the mode controller 252 based on one or more of the gas turbine engines 10A, 10B failing to reach or maintain a dry motoring threshold speed 276 for a predetermined time limit 278. In some embodiments, the dry motoring threshold speed 276 is dynamically adjusted to match a dry motoring profile. For example, the value used as the dry motoring threshold speed 276 can change over time as dry motoring continues such that the dry motoring threshold speed 276 need not be statically defined. In some embodiments, the engine data 209 includes a currently targeted speed for either or both engines 10A, 10B when dynamic adjustments to the dry motoring threshold speed 276 are permitted. In other embodiments, the FADECs 102A, 102B each self-report to the speed monitor 254 an indication of whether each respective engine 10A, 10B is achieving respective targeted speeds/performance during dry motoring. Upon receiving the switch to single engine mode command 274, the mode controller 252 can switch the single/multi engine dry motoring mode 205 from the multi-engine dry motoring mode to the single engine dry motoring mode and disable dry motoring in all but one of the engines 10A, 10B.

FIG. 7 is a flow chart illustrating a method 300 for multi-engine coordination during gas turbine engine motoring in accordance with an embodiment. The method 300 of FIG. 7 is described in reference to FIGS. 1-6 and may be performed with an alternate order and include additional steps. At block 302, a controller (e.g., one of the engine control interfaces 105A, 105B) receives at least one temperature of gas turbine engines 10A, 10B in engine data 209. The at least one temperature can be a measured core engine temperature (e.g., T₃ or T_(core)), an oil temperature (e.g., T_(oil)), and/or an ambient temperature (e.g., T_(amb) or T₂). The engine data 209 can include a number of other parameters used to determine a motoring mode 205.

At block 304, the controller (e.g., one of the engine control interfaces 105A, 105B) determines motoring mode 205 as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on the at least one temperature of a plurality of gas turbine engines 10A, 10B. The motoring mode 205 can also be determined based on a plurality of performance parameters that are based on one or more of: an ambient condition, performance limitations of a compressed air source 114 and each air turbine starter 120A, 120B driven by the compressed air source 114, engine drag, and parasitic factors. Performance parameters can be determined based on one or more of: an ambient air temperature, an ambient pressure, and an oil temperature. The compressed air source 114 can be an auxiliary power unit 113 of the aircraft 5, a ground cart, or a cross engine bleed.

At block 306, dry motoring is initiated based on the motoring mode 205. Initiation of dry motoring can be performed by communicating the motoring mode 205 to either or both of the gas turbine engines 10A, 10B, for instance, on digital communication bus 106 to FADECs 102A, 102B. For example, if either or both of the engine control interfaces 105A, 105B determine that the motoring mode 205 is the single engine dry motoring mode, only a targeted controller (FADEC 102A or 102B) may be requested to initiate dry motoring without explicitly sharing the motoring mode 205 with either FADEC 102A or FADEC 102B. Similarly, if the motoring mode 205 is determined locally at FADEC 102A, the determination to perform single engine dry motoring for gas turbine engine 10A need not be shared with FADEC 102B. Dry motoring can be inhibited under certain conditions, such as when the aircraft 5 is not on the ground (e.g., a weight-on-wheels indication is set for dry motoring to be performed on either engine 10A, 10B).

At block 308, a speed (e.g., N2 or NS) of each of the gas turbine engines 10A, 10B can be monitored when dry motoring is active. At block 310, a switch from the multi-engine dry motoring mode to the single engine dry motoring mode can be commanded based on one or more of the gas turbine engines 10A, 10B failing to reach or maintain a dry motoring threshold speed 276 for a predetermined time limit 278. The dry motoring threshold speed 276 can be dynamically adjusted to match a dry motoring profile.

Accordingly and as mentioned above, it is desirable to detect, prevent and/or clear a “bowed rotor” condition in a gas turbine engine that may occur after the engine has been shut down. As described herein and in one non-limiting embodiment, the FADECs 102A, 102B (e.g., controller 102) and/or engine control interfaces 105A, 105B may be programmed to automatically take the necessary measures in order to provide for a modified start sequence without pilot intervention other than the initial start request. In an exemplary embodiment, the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B each comprises a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm and/or algorithms that control the start sequence of the gas turbine engine. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments of the disclosure can be implemented through computer-implemented processes and apparatuses for practicing those processes.

While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

What is claimed is:
 1. A system for multi-engine coordination of gas turbine engine motoring in an aircraft, the system comprising: a controller operable to determine a motoring mode as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on at least one temperature of a plurality of gas turbine engines and initiate dry motoring based on the motoring mode.
 2. The system as in claim 1, wherein the at least one temperature is a measured core engine temperature or an oil temperature.
 3. The system as in claim 1, wherein dry motoring is inhibited when the aircraft is not on the ground.
 4. The system as in claim 1, wherein the motoring mode is further determined based on a plurality of performance parameters that are based on one or more of: an ambient condition, performance limitations of a compressed air source and each air turbine starter driven by the compressed air source, engine drag, and parasitic factors.
 5. The system as in claim 4, wherein the performance parameters are determined based on one or more of: an ambient air temperature, an ambient pressure, and an oil temperature.
 6. The system as is claim 1, wherein the compressed air source is an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.
 7. The system as in claim 1, wherein the controller is further operable to monitor a speed of each of the gas turbine engines when dry motoring is active and switch from the multi-engine dry motoring mode to the single engine dry motoring mode based on one or more of the gas turbine engines failing to reach or maintain a dry motoring threshold speed for a predetermined time limit.
 8. The system as in claim 7, wherein the dry motoring threshold speed is dynamically adjusted to match a dry motoring profile.
 9. A system of an aircraft, the system comprising: a compressed air source operable to supply compressed air; a first engine system comprising a first gas turbine engine, a first air turbine starter, a first starter air valve, and a first controller operable to command the first starter air valve to control delivery of the compressed air to the first air turbine starter during motoring of the first gas turbine engine; a second engine system comprising a second gas turbine engine, a second air turbine starter, a second starter air valve, and a second controller operable to command the second starter air valve to control delivery of the compressed air to the second air turbine starter during motoring of the second gas turbine engine; and at least one engine control interface operable to determine a motoring mode as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on at least one temperature of the first and second gas turbine engines and initiate dry motoring based on the motoring mode.
 10. The system of claim 9, wherein dry motoring is inhibited when the aircraft is not on the ground.
 11. The system of claim 9, wherein the motoring mode is further determined based on a plurality of performance parameters that are based on one or more of: an ambient condition, performance limitations of a compressed air source and each air turbine starter driven by the compressed air source, engine drag, and parasitic factors.
 12. The system of claim 9, wherein the compressed air source is an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.
 13. The system of claim 9, wherein the at least one engine control interface is further operable to monitor a speed of the first and second gas turbine engines when dry motoring is active and switch from the multi-engine dry motoring mode to the single engine dry motoring mode based the first or second gas turbine engine failing to reach or maintain a dry motoring threshold speed for a predetermined time limit.
 14. The system of claim 13, wherein the dry motoring threshold speed is dynamically adjusted to match a dry motoring profile.
 15. A method for multi-engine coordination during gas turbine engine motoring, the method comprising: receiving, by a controller, at least one temperature of a plurality of gas turbine engines; determining, by the controller, a motoring mode as a selection between a single engine dry motoring mode and a multi-engine dry motoring mode based on the at least one temperature of the plurality of gas turbine engines; and initiating dry motoring based on the motoring mode.
 16. The method as in claim 15, further comprising inhibiting dry motoring when the aircraft is not on the ground.
 17. The method as in claim 15, wherein the motoring mode is further determined based on a plurality of performance parameters that are based on one or more of: an ambient condition, performance limitations of a compressed air source and each air turbine starter driven by the compressed air source, engine drag, and parasitic factors.
 18. The method as in claim 15, wherein the compressed air source is an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.
 19. The method as in claim 15, further comprising: monitoring a speed of each of the gas turbine engines when dry motoring is active; and switching from the multi-engine dry motoring mode to the single engine dry motoring mode based on one or more of the gas turbine engines failing to reach or maintain a dry motoring threshold speed for a predetermined time limit.
 20. The method as in claim 19, wherein the dry motoring threshold speed is dynamically adjusted to match a dry motoring profile. 